Review my RV System

gkerlin · June 2015

Ok - this will be my third RV that I'm putting solar on. My last 5th wheel was 630w and worked perfectly. We did 3 months to Alaska and back and never used a generator. We run the tv, I still work from my rig and we use a fair amount of power. I'm hoping for similar performance out of this one and I'll be very happy.

The Details:

40' 2012 Class A Diesel Pusher.
Residential Fridge
900ah of Lifeline AGM's for house bank
3000w Magnum inverter.

My plan:

8 140W Kyocerra panels
Midnight Classic 150
Wired 2 panels each in series to form a 35v string
4 strings of 2 panels each wired in parallel.

My thought is to create 4 series wired strings. 2 140w panels each string. The 4 strings then would be wired together parallel. Put 2 strings on one side of roof and 2 strings on other side of roof. This way when sun comes from a side of the RV the panels on one side should be in the clear (no shadows). Assume no shading from trees as we avoid that as much as possible. Most of our shading comes from the stuff on the roof.

If I wire all 8 panels parallel up on the roof, then I need to bring a big cable (#2 or #4) down from the roof. That is not preferable. If I wire them all parallel but combine them down below I need to bring 16 conductors down below... That just too many.

This brings me back to my preferred plan of attack:

If I wire them into the 4 series strings on the roof then I could

use the #10 PV wire and bring the 8 conductors(4 pos + 4 neg) down from the roof and combine them inside. About 15' each.
each of the pos leads I would run thru a circuit breaker first then combine them right after
feed that combined (heavier) hot lead to the controller. Which would be in the immediate vicinity.
The neg leads would be combined in a J-box then fed to the controller.
On the controller output side to the batteries I would have another larger circuit breaker. Haven't calculated the exact size - likely around 80amp.
Batteries would be on other side of wall from controller/combiner/circuit breakers.

Any thoughts/concerns on this approach?

Thanks

Greg

BB. · June 2015

What is the battery bank voltage?

What do you really need for an AC inverter--3kWatt is a lot (and can draw 20-40+ Watts just turned on with no loads).

Obviously, shading from stuff on the roof is bad news for solar electric panels (even a little shading on a single panel can knock out the whole string's output). Are the panels going to be mounted float or titled?

-Bill

gkerlin · June 2015

12v battery bank.
The inverter powers the residential fridge, microwave, 4 TVs, Satellite receivers, computers, scanner, office phones etc etc. It is on most of the time. It is actually 2800w with peak output over 3k It pulls about 2amps in standby.
Panels will be flat mounted.

I understand that shading is not good but its fairly unavoidable sometimes on an RV.

BB. · June 2015

Hmm... Assuming a decent 4-5 hours per day of sunlight:

8x140 Watt panels * 0.52 system eff * 4 hours of sun = 2,330 Watt*Hours per day
8x140 Watt panels * 0.52 system eff * 5 hours of sun = 2,912 WH per day

And the inverter:

2 amps * 13.6 volts nominal * 24 hours = 653 Watt*Hours per 24 hour day

That inverter takes ~1/4 to 1/3 of your daily solar power production.

Looking at a smaller AC inverter (Morningstar 300 Watt @ 12 volt TSW) takes something like 6 watts (wire this to your 24 hour per day loads):

6 Watts * 24 hours = 144 Watt*Hours per 24 hour day

For charging your 900 AH @ 12 volt battery bank using 5% to 13% rate of charge:

900 AH * 14.4 volts charging * 1/0.77 panel+controller losses * 0.05 rate of charge = 842 Watt array minimum
900 AH * 14.4 volts charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 1,683 Watt array nominal
900 AH * 14.4 volts charging * 1/0.77 panel+controller losses * 0.13 rate of charge = 2,188 Watt array "cost effective" maximum

8x140 Watt panels = 1,120 Watt array --- For an RV with a large battery bank, not too bad. You always have the genset if you need more charging power on occasion (I like to suggest 10% minimum charging current for an off grid cabin--But many times, people just cannot fit that large of an array on their RV).

Anyway--That is roughly what your system looks like to me from the numbers side. Is this going to meet your needs?

-Bill

gkerlin · June 2015

I am estimating that I need about 2400-2800 WH per day and feel that my proposed system should fill that need.
I do have the option of turning off the inverter at night - especially in the winter in Az when the Refrigerator can hold the cold overnight. That will save me some more ah if needed.
Since the inverter runs the fridge (not a propane fridge) it limits me a bit. That's why I've gone with a much larger array than my last.
Last option is the generator. 8kw

NoGas · July 2015

Hello everyone! Working on system similar to gkerlin's for my MH. Looking for ideas. Proposed configuration as follows: 5 - SolarWorld 285 or 315 watt panels (yes, I have the rooftop space) all wired in one series string to a Midnight 250 Classic charge controller. Right now I have 3 - 4D batts for 630 ah total at 12v but plan is to replace or possibly supplement those with a Balqon.com 9kw lithium pack - http://www.balqon.com/online-store/#...egory=12286435. I have a new MS2812 Magnum inverter/charger, a Samsung RF18 house fridge and a 12 KW PowerTech diesel generator. Coach has no LP gas so uses diesel fired boiler for primary heat supplemented by electric. I also have a Yamaha 2000i generator to charge quietly until the solar system is setup. Recomendations on how I can improve this plan, anyone?

gkerlin wrote: »

I am estimating that I need about 2400-2800 WH per day and feel that my proposed system should fill that need.
I do have the option of turning off the inverter at night - especially in the winter in Az when the Refrigerator can hold the cold overnight. That will save me some more ah if needed. Since the inverter runs the fridge (not a propane fridge) it limits me a bit. That's why I've gone with a much larger array than my last. Last option is the generator.

gerkin - which Magnum inverter-charger do you have? As I understand it, under no load the MS2812 can be set to default to a standby search mode where it uses < 8 watts (similar to the smaller 300 watt inverter mentioned above). When a load is sensed by the MS2812, the inverter instantly turns on and the on-off trigger values can be programmed on the set up page. As to whether or not your proposed system will satisfy your demand, have you seen this spreadsheet? - http://www.jackdanmayer.com/Files/So...frigerator.xls. Play with your numbers and see what you come up with. By the way, the Electrical-Solar page on the JDM site is worth a look.

gkerlin · July 2015

NoGas.. Sorry for the delay. I have a ME 2512. Its a MSW inverter. My old one was a PSW. When I said it pulled 2a - i should have clarified that was when it was inverting. In search mode it pulls about .5a Yes you can set the search mode higher (needs higher loads before it will start inverting) I have my search mode set low because when I want power I want power - regardless of the size of the load. For really small things - cell phone charger for instance - I have a cigarette lighter type inverter. My last system on my 5th wheel was perfect for my needs. I spend a few months in Alaska last summer and never ran the generator once. In Az in the winters I've only had to run the generator a couple of times and that was due to several cloudy days in a row.

The new Class A has higher power needs. It has 4 TVs! I'll disconnect (unplug) TV's and throw breakers to conserve power so that I can try to live off of what my panels produce. My prior system was 630w. This one will be almost twice that. The real difference in power needs between the old and new will be the residential fridge which I expect will pull about 160ah per day. I was averaging about 180ah daily in AZ winters with the old system. This system should just about make up that deficit. If not I have room for more panels and there is always the 8kw diesel generator as a last resort.

I'd question your intention to wire all the panels in series however - unless you can be sure of no shading on all of the panels. If one of your panels gets shaded it will pull down the whole string. In Parallel if one panel gets shaded you only lose that panel.

NoGas · July 2015

Gkerlin - sounds like we are putting together very similar systems. Phantom loads on larger coaches are almost out of control! It is as if manufacturers assume we all are going to be sitting in some shoulder-to-shoulder campground sucking power off a cord. Reducing phantom draws with on-off toggle switches and such (but not cb's) can result in huge power savings. Your shading comment regards series wiring used to be true. However, with the advent of bypass diodes built right into panels it is less of a concern now. Though the english is not so good this web site page explains it: http://www.mpptsolar.com/en/solar-panels-in-series.html. According to SolarWorld specs, their larger Mono panels have 3 bypass diodes. As I understand it, that means each 1/3 of a panel is "hard" shade event protected. In the event shade hits a portion of one panel in a string of panels, only the shaded part of that panels' output is lost (in 1/3 increments). However the rest of the string is not affected as in the past. Of course string voltage will go down slightly (just as it does with a soft shading event for the whole string), but the MPPT controller is designed to accept a wide variation in input voltage and will compensate. A small full-time PSW inverter dedicated to just the refer is not a bad idea. Turning off the defrost/ice making functions is also a huge energy saver when boondocking. If you have the space up top, I would encourage you to install as much capacity as you can. One can never have too much "free" juice for those 4 TV's and iced drinks!

BB. · July 2015

Note that bypass diodes are there to protect the cells against damage (or even fire) if there is shading on the panel.

The bypass diodes do not really help if you have several parallel strings and one panel has partial shading. For example, if you have 2x 30 volt panels in series (vmp~60 volts for the array) and 2x parallel strings... Partial shading on one panel with 3 diodes, then, roughly that panel has -10 volts of output. Or you have one string with Vmp~50 volts and the other string with Vmp~60 volts in parallel. The 50 Volt string is going to supply very little current unless the MPPT controller recacluates the Vmp-array closer to 50 volts--And it is very unclear what Vmp-array voltage the MPPT controller "will choose".

If the controller picks 60 volts, the other 50 volt string will have near zero current. If the controller picks 50 volts, then both strings will produce power, but you will have roughly 20% array power reduction due to one small shadow on one panel.

I have not seen any controller advertise that they have the ability to work with partial shading and the ability to pick the "local maximum" vs absolute max power peak (you can have two to three local peaks with a shaded cell in the above array).

My two cents, reduce shading as much as you can. And assume the worst (that panel/string is out of service while even partially shaded).

-Bill

Iceni John · July 2015

Back to the matter of possible shading sometimes of some of the panels (due to rooftop A/Cs, ventilators, antennas, etc) - why not split the PV array into two separate arrays, with a charge controller for each half? This way if (when?) one array is shaded when the sun is lower during early mornings or late afternoons, the other array will still be unaffected and charging the batteries as best as it can. If one array's voltage or current is different than the other's, the MPPT won't get confused. Also, redundancy is good, especially if you're completely dependent on PV for all your electrical needs - Sod's Law always strikes hard when you least expect it or can least deal with its consequences! I have split my bus's house system into two entirely separate halves for these reasons.

John

NoGas · July 2015

Iceni John wrote: »

...why not split the PV array into two separate arrays, with a charge controller for each half?... I have split my bus's house system into two entirely separate halves for these reasons. - John

Thanks! It appears that is exactly what I am going to have to do, but for reasons unrelated to shading. To "try" different panel configurations I "plugged" them into the online Midnite Classic charge controller String Array Sizing Calculator - http://www.midnitesolar.com/sizingTool/index.php. In the process I found some interesting results. Besides a voltage ceiling, there is also an upper watts limit that these controllers can handle. So even if one stays below the voltage limits of the controller, the watts being generated might exceed that controllers capability. Also, the lower voltage 150 Classic appears to generate more amps than the other higher volt capacity controllers! For those reasons it appears I will need to split my system up into two separate arrays with dedicated (but linked) controllers for each. Would love to go with 6 panels total and if I can figure out a way to fit them into the available real estate I certainly do that. Wow! 8 panels on a 40 footer - I'm impressed! Staying with series wiring instead of parallel should avoid "controller confusion" and the two separate arrays will help with the shading aspect, I hope!... Stay tuned

westbranch · July 2015

here is a VW beetle bus with 10 panels on it FYI... interesting install

http://www.xconomy.com/boston/2014/11/21/vecna-cto-builds-solar-powered-volkswagen-bus-and-it-actually-works/

Review my RV System

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